The present invention relates to anti-lock braking systems and in particular to an anti-lock braking system of the pump-back type which utilizes a common source control valve for each braking circuit of the system and a modulating valve pair including a build/hold and a decay valve for each control channel of the system.
Anti-lock braking systems are now well known. Contemporary systems, which have been variously denominated as anti-lock, anti-skid and adaptive braking systems, can be roughly categorized as including full power systems which utilize a motor driven pump as a boost source and as the primary source of pressurized brake fluid during anti-lock braking system operation, pump-back anti-lock braking systems which use a vacuum boosted master cylinder as a primary source of pressurized braking fluid and a small intermittently operated pump as a source of pressurized braking fluid during anti-lock operation, and mechanical systems in which variously configured inertia devices are used to sense and modulate the application of brake force of the vehicle's wheels. Of these systems, pump-back systems provide the speed and effectiveness of an electronically controlled system, are directly adaptable to existing vacuum boosted braking systems as an add-on system, and are of somewhat lower cost. Pump-back systems have the further advantage in that they provide a substantially conventional braking system using proven components for the vehicle during non-anti-lock braking system operation.
Of primary concern in the continuing development of anti-lock braking systems is the provision of such a system that is substantially failsafe such a system which allows substantial latitude in control algorithms such that factors including pedal feel, system adaptability, cost and space minimization, simplification, and the like can be accommodated with minimal hardware modifications.
Broadly, the present invention is an anti-lock braking system of the pump-back type which provides a novel combination of three-way source control and build/hold valves in combination with two-way decay valves to produce an anti-lock braking system of reduced complexity, increased reliability, and in which electronically controlled valves provide substantial versatility for electronic control of braking forces and other system parameters. In a specific embodiment of the invention, a single three-way source control valve is provided for each hydraulic circuit of the braking system. This valve is connected in series with a three-way build/hold valve and a two-way decay valve provided for each control channel of the system. A replenishment pump including a sump and an accumulator is provided wherein the sump and accumulator volumes are matched to pump capacity to limit brake fluid depletion. The build/hold valves also function as high pressure relief valves without additional component count. The system provides for pressure decay, hold, and build. The systems existing hydraulic system split is fully maintained. The system minimizes pedal pumping during cycling, and the valve arrangement provides redundant protection.
It is therefore an object of the invention to provide an improved anti-lock braking system of the pump-back type.
Another object of the invention is to provide such a system utilizing a single source control three-way valve for each hydraulic circuit of the system and a three-way build/hold valve and a two-way decay valve for each channel of the system.
Yet another object of the invention is to provide such a system in which the valves are electronically controllable in sequence and overlapping relationship to effect maximum electronic control of factors including braking force pedal drop and feel, and fluid loss during anti-lock operation.
Another object of the invention is to provide such a system having reduced component count.
Yet another object of the invention is to provide such a system effecting minimal pedal pumping during cycling and wherein matched displacement components limit pressure loss with failed pump conditions.